Where a feed gas is to be subjected to downstream processing, it may often be desirable or necessary to remove certain components from the feed gas prior to such processing. As an example, water, carbon dioxide, or hydrocarbon compounds may be present in a feed gas, for example, natural gas or air where the feed gas is to be subsequently treated in a downstream process, for example, a membrane process for removal of carbon dioxide or a cryogenic distillation process for air separation. If these materials are not removed, they may cause, for example, fouling or deterioration of equipment in a downstream process or result in other disadvantages in the downstream process.
Many methods for removing an undesired component from a feed gas involve adsorption of the undesired component onto a solid adsorbent by passing the feed gas through an adsorbent bed. One such method is Temperature Swing Adsorption (TSA). Conventional TSA systems are described in, for example, U.S. Pat. No. 5,846,295 and U.S. Pat. No. 7,022,159. In a simple TSA system, two adsorbent beds are employed in a parallel arrangement with one bed being operated for adsorption while the other bed is being regenerated. An adsorption bed is said to be “on-line” during an adsorption step and “off-line” while being regenerated. Alternatively, a TSA system may contain three adsorbent beds, with one bed being operated for adsorption while the other two beds are being regenerated. It is also common practice to use two or more adsorbent beds in parallel “on-line” adsorption in order to accommodate higher feed gas flow rates. In all of these TSA systems, the absorption beds are periodically switched between adsorption and regeneration during an operating cycle.
In a TSA process, adsorption of undesired components is typically promoted by low temperatures. Once adsorption of an undesired component has been carried out in a first adsorbent bed at a low temperature, the flow of feed gas is switched from the first adsorbent bed to a second adsorbent bed. The first adsorbent bed is then regenerated. Regeneration typically includes a heating step. In the heating step, the first adsorbent bed is exposed to a hot regeneration gas at a high temperature which strips the adsorbed materials from the first adsorbent bed and so regenerates it for further use. Typically, the hot regeneration gas is a waste stream or other gas from a downstream process or else a portion of the product gas. Because adsorption is promoted by low temperatures, regeneration typically also includes a cooling step subsequent to exposing the first adsorbent bed to the hot regeneration gas. In the cooling step, the first adsorbent bed is subjected to a flow of cooling gas, which cools the first adsorbent bed down from approximately the high temperature of the hot regeneration gas to approximately the low adsorption temperature in readiness for a subsequent adsorption step.
The operating cost of a TSA system is largely comprised of the cost of the heat required to regenerate an adsorbent bed. The heat for regenerating an adsorbent bed is typically supplied by a regeneration heater. In order to reduce the operating cost of a TSA system, it is desirable to recover heat from an adsorbent bed after it has been exposed to a hot regeneration gas. Preferably, the heat from the adsorbent bed is recovered during the cooling step. In order to accomplish such heat recovery, a TSA system must comprise at least three absorption beds, with two of the adsorption beds being off-line and operated for regeneration at any given time. One of the absorption beds being regenerated will be in the heating step and the other adsorbent bed being regenerated will be in the cooling step. In this arrangement, an effluent gas from the adsorbent bed in the cooling step is used as the feed gas for the adsorbent bed in the heating step and hence, the heat from the adsorbent bed in the cooling step is recovered. This arrangement thus reduces the heating requirement of the regeneration heater and thereby reduces the operating cost of the TSA system. However, the capital and equipment cost of such a TSA system is increased relative to a simple TSA system because of the greater number of adsorbent beds required—a minimum of three adsorption beds compared to only two adsorption beds for a simple TSA system. Therefore, a TSA system and TSA process in which operating costs are reduced by heat recovery, without increasing the number of adsorbent beds and capital cost required is desired.